Slow Linear Voltage Shifter: By pushing one button, the output voltage will rise or fall. By releasing the button, the output keeps its actual potential for a longer time. The rising/falling times are set by the 5M resistors (or the capacity). Use an operational amplifier with FET-Inputs to avoid that the condensor is discharged by the OP. These circuits may be driven with logic TTL, CMOS or digital receiver units etc. The INVERTER could also be replaced by a logic inverter (CMOS 4049). By adding one NPN-Transistor, you can alter the speed of a low-power motor.

Motor Speed Control in Two Directions, Flank/Zero-Crossing Detection, Preamp. with Comparator:

Simple Speed/Direction Control with Quick-Start for small DC-Motors : (preliminary theoretic circuits)
Close around the potentiometer's center position, the OP acts as a HIGH-GAIN amplifier (a>1). Outside this angle area, the OP's amplification is lower (ideal: a=1). This involves that the output voltage rises faster to the motor's minimal starting voltage. It is recommended to make some tests to define the appropriate values for the Zener and the resistances.

In all these circuits, both Zener-diodes may be replaced by two or more diodes like this :

Simple Speed/Direction Control with Quick-Start and STOP-AREA for small DC-Motors : (preliminary theoretic circuits)
With a pair of output transistors the potentiometer gets a small stop area around its center position. This circuit allows the use of a simple standard operational amplifier as the transistors work as an additional booster.

Simple BRIDGE Speed/Direction Control with Quick-Start for small DC-Motors : (preliminary theoretic circuits)
Like in the above circuits, two pairs of output transistors may be added to obtain a stop area or to amplify the output of a standard operational amplifier.

Precision Half Wave/Full Wave Rectifier:

High Voltage, High Current Circuits:

Fast Digital/Analog Converter, Voltage Doubler: